Liquid propellant



July O, 1956 D. R. cARMoDY LIQUID PROPELLANT 2 Sheets-Sheet 1 Filed June 24, 1952 a., Jammin/w31 III o., sanlvysdwsl INVENTOR.

Don Carmody ATTO/WYE;

July 10 1956 D. R. CARMODY 2,753,683

LIQUID PROPELLANT Filed June 24, 1952 2 Sheets-Sheet 2 TARTEI? OX/D/ZER Fig. 2

/IVERT GAS INVENTOR.

Don f?. Garmady polar and ksub-polar regions may be `a freezing point of about 45 C. 49" ing point of water. The

-pincreases the freezing point. l i .can bey obtained by the use of aqueous potassium nitrate United States Patent O LIQUID PROPELLANT Don R. Carmody, Crete, Ill., assignor to Standard Oil Company, Chicago, Ill., a corporation of Indiana Application June 24, 1952, Serial No. 295,215 6 Claims. (Cl. 60--35.4)

This invention relates to reaction propulsion. More particularly, it relates to a nitric acid oxidizer for use in reaction propulsion. Still more particularly, the invention relates to a composition consisting essentially of substantially anhydrous nitric acid and a nitroparain, which composition is characterized by a freezing `point lower than that of the nitric acid component alone.

Reaction propulsion is now being used for many purposes. For uses such as missiles and for assisting the take-off of airplanes, the preferred form of reaction propulsion` requires a self-contained fuel system, i. e., one

not dependent on atmospheric oxygen, i. e., rocket propulsion.

The fuels used for this purpose may be liquid or solid. The liquid fuels are divided into the monopropellants and the bipropellants. The monopropellants decompose to give hot materials which provide the driving force for the rocket; the best known monopropellant is nitromethane. The bipropellent fuels consist of a fuel proper and an oxidizer; examples of these fuels are alcohol, gasoline, aromatic amines, hydrazine, etc.; examples of oxidizers are liquid oxygen, 90% hydrogen peroxide, white fuming nitric acid, red fuming nitric acid, etc.

In the bipropellent system the fuel and the oxidizer are injected separately and simultaneously into the combustion chamber of the rocket motor; ignition means may be supplied to initiate the combustion or the combustion may be spontaneousrthe products of decomposition resulting ,from the reaction of the fuel and the oxidizer are discharged through an orifice, provided at the exit end of the combustion chamber, to produce the driving force. The spontaneous combustion or the self-ignition of an oxidizer and a fuel is `commonly called a hypergolic reaction; the fuel which has these properties is called a hypergolic fuel.

The temperature at the earths surface in the temperate ranges may be as low as 40 C., and in the as low as 54 C. (,-.65 F.) or even lower. The temperatures encountered `by airplanes at high altitude are often as low as 73 C. `100 F). Thus a rocket may have to be started into operation with the oxidizer and fuel at a temperature as low as or lower than 54 C. 65 F.).

Nitric acid is the most common oxidizer. Particularly useful are white fuming nitric acid (WFNA) and red fuming nitric acid (RFNA). However, nitric acid has the `disadvantage of a relatively high freezing point. Nitric acid that is essentially free of water has a freezing point of 42 C. 44 F.). Commercial grade WFNA, which usually `contains between 2 and 3% of water has F.). The freeznitric acid is lowered by the addition of elfect of water in the freezing point of nitric acid is .as follows: freezing point of 50 C. 58 10% Water, 63 C. 8l R); additional Water A still lower freezing point or sodium nitrate. Thus a mixture consisting of 92% 2,753,683 Fatented July 1G, i956 llC 2 acid, 4% water and 4% nitrate has a freezing point of about 68 C. 90 F.).

The additionof water or aqueous salt solution to the nitric acid is undesirable because the total weight of the oxidizer is increased and simultaneously the energy content of the system is decreased. When a hypergolic system is being used, the presence of more than about 5% of water cannot be tolerated because the hypergolic activity is lowered below the point of reliable operation.

An object of this invention is to provide a reaction propulsion method utilizing a self-contained fuel system. Another object is to provide a nitric acid oxidizer that has a freezing point below that of substantially anhydrous nitric acid and preferably below about 54 C. i-'65 F.). Still another object is a nitric acid oxidizer having a desirably low freezing point and of desirably high activ-V ity in a hypergolic bipropellent system. Yet another object is to provide a monopropellent rocket fuel consisting essentially of a substantially anhydrous nitric acid oxidizer and a defined nitroparafn, which fuel has a desirably low freezing point. A further object is to provide a low freezing point nitric acid oxidizer which has good activity in a hypergolic bipropellent rocket fuel system and has a favorable effect on the energy content of the system. A particular object is a nitroparaiin monopropellant which is not susceptible to flashback from the combustion chamber into the fuel tank.

The above objects and other objects which will become apparent in the detailed description have been achieved by the use of a composition consisting essentially of substantially anhydrous nitric acid and at least one mononitroparain which contains from 1 to 4 carbon atoms, which nitroparafin is present in an amount sufcient to form a mixture having a freezing point below that of the nitric acid component. Preferably, the mononitroparan is present in an amount such that the composition contains an excess of oxygen over the amount needed to react with the oxidizable elements present in the composition, i. e., the nitroparain is present in an amount substantially less than that needed to produce a composition that is stoichiometrically balanced with respect to oxygen.

Figure 1 is a freezing point diagram of several compositions of this invention.

Figure 2 shows a schematic layout of the combustion chamber and fuel system of a bipropellent rocket unit.

The term nitric acid as used herein is intended to include HNOa and aqueous solutions of HNOa. Also, it is intended to include HNOs which has been fortified with nitrogen tetroxide N204 and aqueous solutions of HNOa which have been fortified with N204. The preferred acids are white fuming nitric acid, which in the commercial grade normally contains less than about 3 weight percent of water; and red fuming nitric acid, which normally contains about 3% of water and between about 5 and 20 weight percent of N204.

The usefulness of nitric acid as an oxidizer, in conjunction with a hypergolic fuel, decreases rapidly as the Water content of the acid increases. A practical limit on the water content is about 5%. It is impractical to use acid containing more water because the ignition delay becomes excessive at low temperatures, i. e., time for the combustion to begin after contacting the oxidizer and the fuel. The term substantially anhydrous nitric acid is intended to include the defined nitric acids wherein the water content is not more than about 5 weight percent.

The mononitroparains, such as, nitromethane, nitro- `ethane and nitropropane, have been used heretofore as the use of monomtroparafns is their tendency to transmit the decomposition from the combustion chamber into the fuel line and back into the fuel tank, i. e., the so-called flashback. Many methods of overcoming this ashback have been introduced. None have been particularly successful heretofore. Nitroethane and nitropropane have desirably low freezing points of about 90" C. 130 F.). Nitromethane freezes at the relatively high temperature of 28.5 C. l9 F.). Y

Dinitromethane decomposes spontaneously at about 100 C. The other dinitroparains are almost as susceptible to thermal decomposition. The polynitroparafns are so susceptible to spontaneous detonation at low temperatures as to be unusable.

It has been found that the mononitroparaiiins which contain from 1 to 4 carbon atoms are the useful members of the nitroparaflin group for the purposes of this invention. ,The preferred nitroparaffin is nitromethane. nNot only the pure compounds but also the commercial purity materials, i. e., technical grade are included within the scope of the invention.

It has been discovered that a composition having a desirably low freezing point can be prepared by admixing substantially anhydrous nitric acid and at least 1 mononitroparafn which contains from 1 to 4 carbon atoms. The freezing point of a substantially anhydrous nitric acid-nitromethane mixture is very surprisingly lower than the freezing point of either component.

The defined mononitroparains and substantially anhydrous nitric acid are compatible in all proportions. The compositions of this invention are quite stable to shock and to storage in light. A composition consisting of about 33% technical grade nitromethane and 67% commercial WFNA was stored in a clear glass bottle exposed to light at a temperature of about 25 C. for some ten weeks; freezing point determinations indicated substantially no change in composition; the color of the mixture also remained substantially unchanged. The shock stability of the same composition was tested by the use of a dropping ball test. A 2 kilogram steel ball was dropped from a height of 160 centimeters onto the sample which was supported on a steel anvil; no detonation occurred. A sample of TNT exploded when the ball was dropped from a height of only 90 centimeters.

In Figure 1 are presented freezing point'curves for three nitroparain-nitric acid systems. The white furning nitric acid used in determining the data shown in Figure 1 contained 2.6% water and 0.06% N204. The nitroparal'lins were technical grade as purchased from commercial suppliers.

The complete freezing point diagram for the system WFNA-nitromethane is shown as curve A. The presence of some impurities in the nitromethane is shown by the fact that the freezing point of the material used was 31 C. 24 F.) as compared with the freezing point of the pure material of 28.5 It is believed these impurities are mainly nitro compounds.

Curve A shows that the freezing point of this particular WFNA was lowered to below 54 C. by the addition of 15 volume percent of point of the mixture remained below about 54 until the mixture contained more than about 72 volume percent of nitromethane. A composition having a freezing point below about 73 C. l00 between about 28 and 40 volume percent of nitromethane.

Curve B shows part of the system WFNA-nitroethane and Curve C shows part of the system WFNA-nitropropane-2. Compositions having a freezing point below about 54 C. 65 F.) are obtained when the composition contains more than about 20% of nitroethane and more than about 25% of nitropropaneLZ, respectively. A composition which has a freezing point of about 73 C. 100 F.) is obtainable by using about 36% of nitroethane or about 40% of nitropropane-Z.

Assuming that the system nitric acid-nitroparain decomposes to form nitrogen, water and carbon dioxide, the

nitromethane and the freezing F.) is obtainable by using v400 C. (752 F.).

4 stoichiometric composition of a nitric acid-nitroparafn mixture can be calculated. By stoichiometric composition, it is to be understood that the composition is balanced with respect to oxygen content. The stoichiometrically balanced compositions have been calculated for nitromethane, nitroethane and nitropropane when using commercial grade WFNA as the oxidizer.

It has been found that mixtures which are in oxygen balance or which are oxygen-deficient are undesirable as rocket propellants because Vof a tendency to flashback It is preferred that the mixture contain less nitroparain than the amount needed to produce a composition that is stoichiometrically balanced with respect to oxygen.

It has been found that the activity of the composition of this invention toward hypergolic fuels decreases slowly with increase in nitroparaflin content. The molecular weight of the nitroparafn has an effect on the hypergolic activity of the composition; the higher M. W. materials have less activity. The effect of the nitroparan on the activity of the nitric acid is more apparent as the temperature of the fuel is lowered. At temperatureson the order of 54 C. 65 F.) it has been found that the effectiveness of the mixture drops to zero before the amount of nitroparafn in the mixture reaches the oxygen balance point. 'It is preferred to use the nitromethane containing composition.

In order to obtain a composition which has a freezing point below about 54 C. 65 F.) and also to obtain acceptable eectiveness toward hypergolic fuels at low temperatures, it is necessary to have present in the composition: between about 15 and 50 volume percent of nitromethane; or between about 20 and 45 volume percent of nitroethane; or between about 25 and 35 volume percent of nitropropane. It is to be understood that the remainder of the composition in each instance is essentially substantially anhydrous nitric acid. Of course it is possible to use mixtures of the defined mononitroparains.

It has been found that the compositions of this invention can be used as monopropellants. The nitropararin-acid composition can be ignited by heating the composition, by the use of a spark plug, a glow plug, or a hot surface. The ignition means is needed only to start the decomposition; very quickly the walls of the combustion chamber become quite hot and the hot walls plus the mass of hot gases in the chamber ignite the mixture as .it enters the chamber. To illustrate, nitromethane begins to decompose at a rapid rate at a temperature of about 350 C. and spontaneously decomposes at a temperature of about The composition should contain substantially less than the amount of nitroparan needed to produce a stoichiometrically balanced composition; this is to prevent destruction of the unit by flashback and decomposition of the fuel in the fuel tank.

Although the composition of this invention can be used as a monopropellant, it is preferred to utilize it as the oxidizer in a bipropellent rocket unit using a hypergolic fuel. Figure 2 illustrates the use of the composition of 'this invention in such a unit which shows schematically the fuel system and the combustion chamber. This type of unit is suitable for use as the driving means for an air-to-airV missile or for assisted take-off of airplanes. In Figure 2, vessel 11 contains a quantity of inert gas under dizer.

avenues the gas from line 1n forces the oxidizer through line 18, lthrough solenoid actuated throttling valve 19, through line 21, and through injector 22 into combustion chamber 23. Combustion chamber 23 is provided with an exit orifice 24. Vessel '26 contains the main supply of fuel. The `gas pressure forces the fuel out of vessel 26 through line 27, through solenoid actuated valve 28, `and through line 29 to vessel 31. Vessel 31 may be used to contain a special starter fuel or an additional amount of the main fuel. The pressure in line 29 forces the contents of ves- Sel 31 through line 32, through `solenoid actuated throt- Afling valve 33, `through line 34 and through injector 36 into combustion chamber 23. The injectors 22 and 36 are s0 arranged that the streams of liquid violently impin'ge and 'thoroughly intermingle. The reaction of the fuel and the `oxidizer results in the generation of a large volume of very hot gases which pass out of the combustion chamber through oritice 24; the reaction from this expulsion of gases drives the rocket.

For ordinary velocity operations, only one fuel is used, herein triethyl trithiophosphite is used, and lbo'th vessels 26 and 31 will contain this fuel and valve 28 will be in the open position. The oxidizer consists of a mixture of 30% nitromethane and 70% of commercial WFNA. This particular mixture is used because of its freezing pointof below 80 C. (-1l2 F.).

The ratio, on a weight basis, between the oxidizer mixture and the fuel may be between about 1.5 and 5.0. In this example 3 lbs. of oxidizer composition are used per pound of fuel. The missile is launched by activating the soleuoids on valves 19A and 33. The oxidizer and the fuel are forced into the combustion chamber by the pressure of helium gas. Combustion takes place almost instantly and the rushof hot gases through orifice 24 hurtl'es `the missile toward .the target.

The walk .of the combustion `chamber become very hot from the heat of the burning gases generated by the reaction of the fuel and the oxidizer. This hot surface, and the mass of hot gases in the chamber, has a pro# nounced favorable eiec't on the self-ignition characteristics of the fuel and oxidizer. Many fuels which are non-hypergolic at the temperature existing in the fuel tank of the rocket unit are rapidly hypergolic .in the extremely hot combustion chamber. For economy of operation, a fuel that is hypergolic at very low temperatures may be `used `to initiate the combustion in and to start the cold reaction motor; the use of this starter fuel may be continued until the hot gases generated have heated the cornbustion chamber `to a high temperature; at vthis point the flow of the starter fuel can be stopped and a cheaper, although not as `highly hypergolc, or even a non-hypergolic fuel, can -be utilized for the continuous operation of the reaction motor.

The use of a starter fuel in conjunction with another type of main fuel is particularly advantageous when very high velocities are necessary. Certain fuels whose decomposition products are of relatively low molecular weight are used for high velocity purposes because of the Very high thrust developed by these fuels. An air-to-air missile usually has a relatively short combustion chamber and this fact limits the fuels that can be used for high thrust operation. Turpentine is an excellent high thrust fuel for this use. The turpentine is stored in vessel 26 and valve 28 is closed. About 4 lbs. of nitromethane-WFNA oxidizer are used per pound of turpentine. Trimethyl tri thiophosphite is used as the starter fuel and is stored in vessel 31. Only enough starter fuel to heat up the combustion chamber is needed; in this case 0.1 second of operation. The missile is launched by activating the solenoids in valves 19, 28 and 33. The turpentine forces the trimethyl trithiophosphite into the combustion chamber where the oxidizer and trimethyl trithiophosphite ignite and heat up the chamber. Without interruption, the turpentine follows into the heated chamber and burns to give the very high thrust reaction.

6 The following tests illustrate the results obtainable with the vcompositions of this invention when used as oxidizers for hypergolic fuels.

Test I in this test the ignition characteristics of the oxidizer compositions were studied using a drop test method. This' method utilizes a test tube, 1 in. x 4 in., containing l ml. of oxidizer.` The lel is added dropwise into the test tube by means of a syringe calibrated in 0.01 ml. markings. Usually 0.1 ml. of fuel is added per test; however, the fuel usage may vary between 0.01 and 0.2 ml. per inl. of oxidizer. Low temperature tests were carried out by cooling the test tube and the oxidizer contained therein to the desired temperature by means of a Dry Ice-chloroform bath; a drying tube inserted into the top of the test tube excluded moisture. The fuel was cooledV separately to the desired temperature. By supercooling, it was possible to carry out tests at temperatures below the freezing point of the fuel and of the oxidizer. The time elapsing between the addition of the fuel to the oxidizer and ig*- nition thereof-the ignition delay-was determined vis'- ually as either: extremely short, very short, short and ignition. An extremely short ignition delay corresponds to substantially instantaneous ignition.

vIn `this test the hypergolic fuels were triethyl trithiophosphite and furfuryl alcohol. Tests were run using WFNA containing 2.6% Water alone and a mixture of 16.6 volume percent nitromethane and 83.4 volume percent ofthe above WFNA.

(a) In these runs the oxidizer was WFNA alone.

, Minimum Fuel Hypergolic Igni ion Tempera- De ay ture, F.

Furiuryl alcohol.; Below 73. I iti` Triethyl trithiophosphte.. -..do glgmon.

NOTE.-0xidizer was frozen in these runs.

(b) In these runs the oxidizer was the nitromethaneacid composition.

Minimum Fuel Hypergolie Ignition Tempelgatue, Delay Fururyl alcohol Below-73.- Ve sh t. Trlethyl trithiophosphite ....,dm l313mm Test 2 In this test the effect of variation in the nitroethne content of 4a mixture of nitroethane and commercial WFNA was studied using the drop test described above. In these runs 0.4 ml. of oxidizer was used and the fuel was a mixture of methyl, ethyl, propyl and some butyl trithiophosphites; the mixture had an approximate average molecular weight of 214. The runs were carried out at two temperatures. Results of these runs are shown For a more accurate determination of the eifect of nitroparan on the usefulness of the nitric acid as the oxidizer forhypergolic fuels, a series of runs were conducted using an apparatus which permitted determination of ignition delay in terms of milliseconds. The ignition delay herein is the length of time elapsing between mixing the fuel and the oxidizer and the start of the spontaneous combustion reaction. These runs were carried out at different temperatures. The fuel used was the mixed trithiophosphite described in Test 2. The nitric acid was commercial WFNA. For purposes of comparison, results lare given using WFNA alone. It is pointed out that for military purposes ignition delays of about 100 milliseconds are acceptable when the combustion chamber and bipropell-ants are at a temperature of about 65 F. The results are given below:

This test shows clearly that the nitromethane and nitroethane containing compositions of this invention provide oxidizers of satisfactory activity when utilized with hypergolic fuels, which oxidizers are of satisfactorily low freezing point.

I claim:

1. A composition which consists essentially of between about 60 and 72 volume percent of an acid selected from the class consisting of white furning nitric acid and red fuming nitric acid, wherein said acid contains not more than about Weight percent of water, and between about 40 and v28 Volume percent of nitromethane, which com# position is characterized by a freezing point below about 73 C.

2. A composition of matter which consists essentially of substantially anhydrous nitric acid and at least one mononitroparain selected from the class consisting of nitromethane, nitroethane and nitropropane, which composition is characterized by having a freezing point below about 54 C., wherein said composition contains said acid and nitroparain in accordance with the following schedule:

3. A composition of matter which consists essentially vof between about 15 and 50 volume percent of nitrof methane and between about and 50 volume percent of substantially anhydrous nitric acid, which composition is characterized by afreezingpoint below about' 54 C.

4. The composition of claim 3, wherein said acid is white fuming nitric acid which contains not more than about 5 weight percent of water.

' 5. A monopropellant reaction propulsion method which comprises injecting into the combustion chamber of a reaction motor an amount of'a composition consisting essentially of between about l5'and 50 volume percent of nitromethane and between about 85 and 50 volume percent of substantially anhydrous nitric acid, applying an ignition means to thek composition-containing chamber, and discharging the products of combustion ofsaid composition through an orifice provided at the exit end of said chamber.

6. A bipropellant reaction propulsion method suitable for operation at a temperature on the order of 54 C., which method comprises injecting separately and substantially simultaneously into the combustion chamber of a reaction motor triethyltrithiophosphite and an oxidizer consisting essentially ofl between about l5 and 50 volume percent of nitromethane and the remainder essentially substantially -anhydrous nitric acid and discharging the products of the reaction of said triethyltrithiophosphite and said oxidizer through an orifice provided at the eXit end of said chamber.

References Cited in the tile of this patent UNITED STATES PATENTS 2,433,943 Zwicky et al. Jan. 6, 1948 2,573,471 Malina et al. Oct. 30, 1951 2,584,803 Hannum Feb. 5, 1952 2,637,161 Tschinkel May 5, 1953 OTHER REFERENCES Klein: Fuels for Jets, S. A. E. Journal, December 1947, pp. 22-28. (Copyrin Scientific Library.)

Journal of the American Rocket Society, December 1947, No. 72. (Copy in Scientific Library.)

Journal of the American Rocket Society, No. 80, March 1950, pp. 1l-l7 inclusive. (Copy in Scientic Library.) p

Gunn: Journal of the American Rocket Society, No. 22, January-February 1952, pp. 33-38 inclusive-Paper presented at the Fall Session of the American Rocket Society on September 26-28, 1951. (Copy in Scientific Library.)

KillefEer: Fuels for J ets, S. A. E., Scientific American, September 1945, pp. 162-164. (Copy in Scientific Library.) 

6. A BIPROPELLANT REACTION PROPULSION METHOD SUITABLE FOR OPERATION AT A TEMPERATURE ON THE ORDER OF -54* C., WHICH METHOD COMPRISES INJECTING SEPARATELY AND SUBSTANTIALLY SIMULTANEOUSLY INTO THE COMBUSTION CHAMBER OF A REACTION MOTOR TRIETHYLTRITHIOPHOSPHITE AND AN OXIDIZER CONSISTING ESSENTIALLY OF BETWEEN ABOUT 15 AND 50 VOLUME PERCENT OF NITROMETHANE AND THE REMAINDER ESSENTIALLY SUBSTANTIALLY ANHYDROUS NITRIC ACID AND DISCHARGING THE PRODUCTS OF THE REACTION OF SAID TRIETHYLTRITHIOPHOSPHITE AND SAID OXIDIZER THROUGH AN ORIFICE PROVIDED AT THE EXIT END OF SAID CHAMBER. 